The present invention relates to imaging systems and in particular to a high resolution flat panel for radiation imaging and to a compensation circuit for an amplified flat panel for radiation imaging.
Flat panels for radiation imaging have been extensively studied for over ten years, and are well known in the art. Examples of flat panels for radiation imaging can be found in the following patents:
U.S. Pat. Nos. 5,132,541, 5,184,018, 5,396,072 and 5,315,101 assigned to Philips;
U.S. Pat. Nos. 4,785,186 and 5,017,989 assigned to Xerox;
U.S. Pat. Nos. 4,382,187, 4,799,094, 4,810,881, and 4,945,243 assigned to Thomson-CSF;
U.S. Pat. Nos. 5,182,624, 5,254,480, 5,368,882, 5,420,454, 5,436,458 and 5,444,756 assigned to 3M;
U.S. Pat. Nos. 5,079,426 and 5,262,649 assigned to Michigan University;
U.S. Pat. Nos. 5,340,988, 5,399,884, 5,480,810, 5,480,812 and 5,187,369 assigned to General Electric; and
U.S. Pat. No. 5,315,102 assigned to Fuji Xerox.
One type of flat panel radiation imaging system includes a thick amorphous selenium (axe2x80x94Se) film on an array of pixels such as that described in the article entitled xe2x80x9cFlat Panel Detector for Digital Radiology Using Active Matrix readout of Amorphous Selenium,xe2x80x9d by W. Zhao et al., Medical Imaging 96, SPIE Conference, SPIE 2708, February 1996. In this flat panel radiation imaging system, the pixels are arranged in rows and columns with each pixel including a TFT switch. Gate lines interconnect the TFT switches in each row of the array while source or data lines interconnect the TFT switches in each column of the array. The thick amorphous selenium film is deposited directly on top of the TFT switch array and a top electrode overlies the amorphous selenium film.
When x-rays are incident on the amorphous selenium film and the top electrode is biased with a high voltage, electron-hole pairs are separated by the electric field across the thickness of the amorphous selenium film. The holes, which are driven by the electric field, move toward the pixel electrodes (i.e. the drain electrodes of the TFT switches) and accumulate in a storage capacitor in each pixel. This results in a charge being held by the pixel electrodes which can be used to develop an x-ray image.
The charges held by the pixel electrodes are read on a row-by-row basis by supplying gating pulses to each gate line in succession. When a gating pulse is supplied to a gate line, the TFT switches of the pixels in the row associated with that gate line turn on, allowing the signal charges stored in the storage capacitor of those pixels to flow to the source lines. Ideally, the TFT switches of the array should be controlled only by the potential voltage on the gate electrode. However, stray electric fields from the amorphous selenium film and the top electrode, which can be up to 10V/m, can have significant effects on the channel conductance of the TFT switches unless special shielding techniques are used. One such shielding technique is to provide a dual-gate structure in the TFT switches. In these TFT switches, one gate is disposed below the semiconductor channel layer and the other gate is positioned above the semiconductor channel layer. The two gates are electrically connected together. An example of a dual-gate TFT switch is disclosed in xe2x80x9cIEEE Transactions on Electronic Devices-28, No.6, pp.740-743, Jun. 1981xe2x80x9d by F. C. Luo et al.
Also, in medical x-ray imaging systems, signal levels are generally much lower than visible light imaging systems, in order to minimize the exposure of patients to x-rays. Therefore, in order to obtain high resolution, a high signal to noise ratio is extremely important. In order to improve the signal to noise ratio in x-ray imaging systems, amplified imaging pixels for flat panels have been considered such as those described in the xe2x80x9cIEEE Journal of Solid-State Circuits, Vol. SC-4, No.6, pp. 333-342, December 1969xe2x80x9d by S. G. Chamberlain and in the xe2x80x9cProceedings of IEDM""93, pp 575-578, December 1993xe2x80x9d by H. Kawashima et al.
In order to reduce the switch noise caused by parasitic capacitance distributed along the source lines and maximize the signal to noise ratio, a charge amplifier is provided for each column of TFT switches in the pixel array. The charge amplifiers sense the charges on the source lines when a row of pixels is gated and provide output voltage signals proportional to the charges and hence, proportional to the exposure of the pixels to radiation. Unfortunately, by providing a charge amplifier for each source line, two problems result. Firstly, in large format radiation imaging systems which include in excess of one thousand (1000) source lines, the cost associated with the charge amplifiers is significant. Secondly, in high resolution radiation imaging systems that have a small pixel pitch, it is difficult to wire-bond the charge amplifiers to each source line. Accordingly, there is a need for an improved high resolution flat panel for radiation imaging.
It is therefore an object of the present invention to provide a novel high resolution flat panel for radiation imaging and a compensation circuit for an amplified flat panel which obviates or mitigates at least one of the above-mentioned problems.
According to one aspect of the present invention there is provided a flat panel for radiation imaging comprising:
a radiation transducer to be exposed to incident radiation;
an array of pixels on one side of said radiation transducer, each of said pixels including a storage capacitor to store signal charge proportional to the exposure of said radiation transducer to radiation in the vicinity of said pixels;
a plurality of gate lines interconnecting the rows of pixels in said array, said gate lines receiving gate pulses to allow said pixels to be selected on a row-by-row basis;
a plurality of source lines interconnecting the columns of pixels in said array to allow the signal charges held by the storage capacitors of said selected pixels to be sensed, at least one pair of adjacent pixels in each row sharing a source line; and
control means to control selection of the pixels sharing a source line so that the signal charge stored by the storage capacitor of only one pixel of each pair can be sensed by way of a shared source line at a time when said row of pixels is selected.
Preferably, the flat panel has multiple pairs of adjacent pixels in each row that share source lines. In one embodiment, during the first half time period of a gate pulse, the control means biases one pixel of the pairs of pixels sharing a source line to allow the signal charges held by those one pixels to be selected in response to the gate pulse, and during the remaining half time period of the gate pulse, the control means biases the other pixel of the pairs of pixels sharing a source line to allow the signal charges held by those other pixels to be selected in response to the gate pulse.
It is also preferred that the flat panel includes refresh means to refresh the storage capacitors of the pixels after the signal charges held thereby have been sensed. In one embodiment, each row of pixels is refreshed as the next row of pixels is being selected. In a different embodiment, the pixels of the flat panel are refreshed after all of the rows of pixels have been selected.
According to yet another aspect of the present invention there is provided a compensation circuit for use in a high resolution amplified flat panel for radiation imaging comprising:
an amplifier having an input terminal to receive amplified signal charge output on a source line by a selected pixel of said flat panel in response to a gate pulse, said amplified signal charge having a dc bias; and
switch means to connect said input terminal to a potential voltage source when said amplified charge is received, said potential voltage source having a magnitude substantially the same as said dc bias but opposite in polarity to offset said dc bias.
The present invention provides advantages in that the need for a charge amplifier associated with each column of TFI switches in the array is obviated. This is achieved by allowing adjacent pixels in the rows of the array to share a source line and therefore a charge amplifier. The pixels sharing a source line are gated at different times to ensure that the signal charge stored by only one of those pixels is applied to a shared source line at a time to avoid mixing of signal charges and therefore maintain high resolution.